Frequency discriminator circuits



Sept. 30, .1947. M G, CROSBY' n 2,428,264

' FREQUENCY DISCRIMINATOR CIRCUITS FildApril 27,' 1943 2 sheets-Sheet 1 INVENTOR All/@PA Y Ci.' C/msx ATTORNEY Sept. 30, 12947.y

M.G. 'CROSBY FREQUENCY DISCRIMINATOR CIRCUIITS 4 Filed Apri; 27, 194s 2 Sheets-Sheet 2 Tlcia.

}./ I B A INVENTOR l /I/MPAA Y @Fosa x ATTORNEY TLEIEQ. l

Patented Sept. 30, 1947 .Radio Corporation of of Delaware Application April 27 My present invention relates to discriminator circuits for waves of variable frequency, and more particularly to frequency modulated wave energy discriminator circuits.

An importantl object of this invention is the provision of a novel method for converting an impedance characteristic into an output characteristic, wherein the characteristics bear an inverse proportionality relation to each other.'

Another important object of my invention is to utilize the aforementioned conversion method in a frequency discriminator network, the latter being provided with input'terminals to which are supplied'frequency-variable waves, and the network havinga pair of output impedances of respectively complementary amplitude-'frequency characteristics.

Another object of my invention is to provide a method of translating' frequency-variable waves into variable-amplitude waves, the method employing a novel network capable of converting a given amplitude-frequencyv characteristic into a complementary amplitude-frequency characteristic, and the two characteristics being caused to vhave 'a cross-over frequency at thel mean frequency value of the frequency-variable waves.

Another object of the rinvention is to provide a frequency discriminator circuit using a single electron discharge device provided with a pair of output impedances, and the frequencyresponse characteristics of the impedances being of a complementary nature, the output impedances additionally being located in common in the space current path of said device.

Still another object of my invention is to provide a demodulator network for frequency modulated carrier wave energy wherein the discriminator section of the network comprises a pair of parallel electron discharge devices having a common cathode load, the output electrodes of the devices having parallel output impedances, the respective amplitude-frequency' characteristics of these parallel impedances being complementary and havinga cross-over frequency at the m'ean frequency of the frequency' modulated energy which is to be demodulated.

Still'otlier objectsof my invention are to improve gerierally frequency discriminator circuits, and more particularly to provide various modificati-ons of circuitsv employing the fundamental impedance inversion method disclosed in ythis application.

. Other features will -best be understood by reference to the following description-taken in connectioniwithrthe drawing, in which I have IAnriercm a corporation 1943,seria1No, 484,703

l indicated diagrammatically several circuit organizations whereby my invention may be carried into effect.

In the drawing:

Fig. 1 shows a generalized network embodying the invention,

Fig. 2 shows my invention embodied in a frequency `Adiscriminator for frequency-variable signals,

Fig. 3 showsthel complementary amplitudefrequency characteristics of the output impedancesf of the circuit of Fig. 2,

`Fig. 3a illustrates a modified form of impedance `network which may besubstituted for the output impedance of the first electronic section ofthe circuit of Fig. 2,

Fig. 4 illustrates a modified form of the invention,

Fig. 5 shows a further modification of the invention, l

f- Fig. 5a shows the complementary frequency response characteristics for the circuit of Fig. 5.

Referring now to the accompanying drawing, wherein like reference characters in the different iigures designate similar circuit elements, in Fig. 1 there is shown a pair'of electron discharge devices' whose electrodes are conveniently located in a common tube envelope yI'. It is to be clearly understood that thev electronic devices may be located in separate tube envelopes. Assuming that tube I is .of the well known twin triode type, the commoncathode lead I is shown connected to ground by a purely resistive impedance 2 which is unbypassed: The input control grid 3 of the rst electronic section is connected to a source of. alternating current energy which is schematically represented. The source of alternating current energy may be a source of high frequency signals, either amplitude modulated, frequency modulated or phase modulated, and is shown connected between ground and the input grid 3.' The plate 4 of the first electronic section is shown connected back to the +B termina] of the direct current energizing source (not shown) rthrough lead 4', an impedance Z and lead 3". At'least a portion of this impedance load isA to be understoodI as being conducted for the direct current flowing through the closed space-current path including the space current of the first electronic section.

, The second electronic section has its control grid 5 connected by lead 5 to the grounded end of cathode resistor 2. The plate 6 is connected to `the -l-B terminal through'a lead Wand a purely; resistive impedance 'Lv The plate end of impedance Z is connected by a coupling condenser 8 to an output circuit, while condenser 9 couples the plate end of output impedance 1 to a second output circuit. In other words, the impedances 'I and Z are the respective output impedances of the two electronic sections. In considering the electrical action of the circuit shown in Fig. 1, itis pointed'outl that the'rstj electronic section of tube I is a so-called cathode follower stage. A

This cathode follower applies alternating voltage between the grid and cathode of the second electron section by applying the voltage to the common cathode resistor 2. The fraction of the voltage applied at grid 3, which appears on the cathode resistor 2, is dependent on the degree of inverse feedback existing in the first electron section. The degree of inverse feedback is dependent on the ratio of the cathode resistance to the impedance Z in the plate circuit of the first section. If the impedance magnitude of load Z is high, a small voltage will be developed across resistor 2 from grid 3. If the impedance magmtude of load Z is low, a high voltage will be developed. Thus, at the frequency corresponding to a high impedance value for load Z, the voltage fed to grid 5 will be low so that the output appearing across resistor 1 will be low. At this same time, the high magnitude of impedance of load Z, appearing in the plate circuit of the rst tube, causes a high output voltage to appear across load Z. Hence, when the value of impedance of load Z is high, the output at condenser 8 is high and that at resistor 'I is low. For the same reason, when load Z is low in impedance value the output at condenser 8 is low, and that at resistor 1 is high. YIt will thus be apparent that the'impedance characteristic of load Z appears inverted in resistor 'I'.

In Fig. 2 I have shown a circuit for utilizing the system of Fig. 1 to provide frequency discrimination. Here the tube I has the input grid 3 yof the rst electron section coupled to the high potential side of the resonant secondary circuit of input transformer T. The resonant primary circuit I I andthe resonant secondary circuit I2 areeach tuned to the mean frequency of the frequency-Variable signal'energy applied to circuit II from any well known source of frequency-variable signals. Such a source of signals, for example, could be intermediate frequency energy of a superheterodyne receiver, whether the'receiver be utilized for amplitude modulated carrier energy or for frequency modulated carrier energy. Of course, in the case of the reception of amplitude modulated carrier energy the discriminator would be employed to produce automatic frequency control (AFC) bias to control the local oscillator in the well known manner. Where frequency modulated signal energy is applied to transformer T-the discriminator network would then function, in cooperation with the usual opposed rectiers, to'provide demodulation for the purpose of producing the original modulation signals, as well as to produce AFC bias for control of the local oscillator. Let it be assumed, for the purposes of this application, that the signals applied to transformer T are frequency modulated (FM) carrier waves, and that the mean frequency of the applied FM energy is at an intermediate frequency value. Each of circuits II and I 2l would then be tuned to the operating I. F. value. l

The plate 4 in this case has connected in circuit therewith a complex impedance 'network is tuned for series resonance to a frequency F2 which is located on the opposite side of the mean frequency Fc. Referring to Fig. 3 the solid line curve Z1 depicts Ythe Amplitude-Frequency" curve of the complex impedance network Z in Fig. 2. It will be'seen that there is an output peak at F1 and a second output peak at F2. The

frequencies F1 and F2 are on opposite sides of Fc, and are located at equal frequency distances from the latter mean frequency.

The plate 6 hasconnected in circuit therewith the-resistor 'l which is'an'aperiodic element. The lead 6 'connects plate 6 to the upper end of resistor 1. The unbypassed cathode resistor 2 is connected from the common cathode lead I to-the grounded side'of input circuit I2. The lead 5' connects grid 5 to the grounded end of resistor 2, and the low potential ends of resistor I and circuitIS, Ill are respectively returned to ground forsignal frequencies by condenser 'I'. The +B terminal is connected to the lower end of resistor l' and by lead 3" to the lower end of coil I3. It will be seen that these connections are generally 'similar to those shown in Fig. 1. The amplitude-frequency characteristic appearing across resistor 'l is represented by the dotted line Z2 in Fig. 3. This amplitude characteristic Z1 is complementary to the corresponding solid lineV characteristic of the complex network Z. In other words,` at frequency F1 the output voltage appearing at resistor 'I is a minimum, while at frequency F2 it is a maximum. 'I'his actionr occurs by virtue of the fact that when the impedance in the plate circuit of the rst electronic section is high, then the output across the cathode load resistor 2 is low. This means that the output at resistor I in circuit with plate 6 of the second electron section is a minimum.

On the other hand, when the impedance of network Z at F2 is low, then the voltage developed across cathode resistor 2 is high and the output at resistor I is also high. It is to be understood that the explanation given in connection with Fig. l, relative to the impedance inversion functions, applies equally well to the circuit of Fig. 2. It will, therefore, be seen that the amplitude characteristics at network Z and resistor 'I in Fig. 2 present to respective rectiers complementary sloping lter characteristics as shown in'Fig. 3; This is the type of network which provides frequency discrimination. In order to produce a'differential rectified voltage output, which is representative of the frequency variations due to the modulating signal upon the carrier, it is necessary to provide a pair of opposed rectifiers I6 and I'I.

These rectiers are shown by way of illustration as being of the diode type. The coupling condenser 8 connects the anode of diode I 6 to the plate endof impedance network Z. The coupling condenser 9 connects the anode of diode I'I to the plate end of aperiodic impedance 1. The cathodes of diodes I6 and Il are connected together byseries-arranged load resistors I8 and I9. Each of the load resistors I8 and I9 is shunted by.;@irrespective-.carrier bypass condenser, andethe cathode of diode I'I is grounded. Thejunctionfof resistorsll 8 and tgis connectedtothejunction of resrectiye direct :current .freturnfresistors k20 .-,and 25;.ea hgofiwhichgreturn.- resistors `isxgassociated withazresnective. onecf :diodes vI 6., and I 1. v n a. s filiere .maybe ,takenzoi from theA cathode end of; resistor Ie-.modulation signal voltage as well Sri;ECFYoltage.. `.'Ihose; skilled in the. vart-.are fully aoquaintedwith the fact that the rectiers I6 and Ilgrespectively Kfunction to rectifygthe instantaneous voltage appearing across the inputjimpedance elements thereof. It.1Wil11,..therefore,f be clear that-when the voltage developed across impedance network Zgis a maximum, the voltage developed at thesamesinstant across impedance will bea minimum. ,g Hence, the differential resultan-t voltage'of ,the rectified voltages developed acrossjeach` ofresistors I8 and I9 is fed tothe utilizing audio network. r

@The slow variations kin differential voltage .are ltered out by the filter network 22 located in the; AFC line leading to the. local oscillator.

At the mean frequency Fc of applied FM` signals thev voltages across. each of network Z and resistor. 'Lare equal. Hence, the differential output voltagefof the rectiers I6 and I1 is zero at Fc. The spacing between Fi and F2 may exceed the overall frequency deviation of the FM signal energy, if desired. If the signal energy applied tofcircuit II vis AM wave energy, then any shi-ft ofthe-carrier frequency of applied signalsfrom Ff.- will provide AFC bias of apolarity and magni-` tude `dependentupon the sense and `extent of frequenoy s l 1ift.` The compleximpedance network g1 may take ,the Aform shown in.Fig. 3a, .if desired. Here, the condenser IIIl and coil I3 are series resonantV to F2.v The coil. I5 is arranged in shunt with circuit I4I3, .and the entire network I3..-IlI-I5 is parallel resonant to F1.

The modication of Fig. 4 is one ywhereinthe plate impedance network in circuit with plate 4 is aktuned circuit 30-3I which is resonant to F2. The plateload in circuit with plate 6 is a second tuned circuit 32-33 resonated to Fc.. As in the case of Athe circuit shown ,in Fig. 2, lead 4' connectsplate 4 to thehighpotential side ofthe impedance network 3I,30, and lead- 3" connects the low-potential side of thelatter to the +B terminal. The common cathode resistor 2 connects the common cathode lead I to ground. The lead B connects'plate 6 to the high potential side of tuned circuit 32, 33, and the lowl potential side of the latter is connected to the +B terminal. Condenser I provides the signal frequency path back to the groundedV end of resistor 2. Here, again, complementary amplitude-frequency characteristics would be presented to the opposed rectiers I6 Vand -I'I. -The -voltage developed across circuit 30-3I is taken olf by condenser' 8, andA fed to rectifler I6. Since this circuit is detuned from the carrier frequency, its amplitude-frequency characteristic is as shown by curve B in Fig. 5a. The voltage across tuned circuit 32-33 is fed to rectifier I'I bycondenser 9.. This output circuit provides a complementary ampli,y tude-frequency characteristic as shown by curve A ofFig. 5a. Hence, the .,well known opposite slopingfrlter. characteristics', appear at respective condensersand 9.Y A l .1n Fig. 5 4I`have shown 'how to'utilize a single electronic section vin place", of *.thertwo electronic sections'oflFig'sfl, 2 and 4.@,l-Iere, the tubegIlU-.is ofi/the screen grid type,`and has its cathode 4I conne'ctedto ground through a parallel resonant circuit comprising: the-coil 42 shuntedby con*- denserz43., Theparallel'resonant circuit is tuned to thefreq'uency Fzwhich is above the center Afrequency Fc.` The signal `:input voltage is schematically represented as 'being'k applied to the inputgrid 50; The A,plate 52 is connected to the +Bterminalcfthe-direct current source through lead 1525: and a relatively low magnitude output resistorxli)` V`-The cathode side of the resonant circuit-.M742 is: connected by lead 4I to a cou. plingrconde'nser B which feeds one of the opposed rectiers. In the sameway the couplingcondenser ,A'.,.whose input electrode is connected to .lead 52', `feeds-the voltage developed across the resistorA GIIv to the second of the opposed rectiers. .Thecomplementarycharacteristics A and B of the impedances 60 andVlI3-42 respectively are shown in Figfa..

.The output across circuit 42-43 isk obtained by .virtue'of` the cathode follower action which dee pends-rupon-the degree of inverse feedback in the tube."'-f-,The degree-of inverse feedback depends uponthe ratioof the impedance of circuit 42`43 to-thelimpedance of resistor 60. Thus, when the impedance -of circuit 42-43 is high, the inverse feedback ishigh and a large output appears across circuit I2- 43: At ythis instant of high inverse feedbackythe amplification of the tube is low so that theY output appearing across resistor Iil is low. It is thus seenthat when the output across circuit 42-43 vis` high, that `acrossl resistor 60 is: low.v Thisiproduces the complementaryampli--v tude-frequency characteristics as shownin Fig. 5a'.f=-These complementary characteristics are producedby the impedance-inverting characteristic existing between the cathode circuit and the plate circuit vofthe tube. Y i Whilel` have indicatedfand described several systemsffor carryingmy invention into effect, it will=be apparent to one skilled in the art that myv-invention is 'by no means limited to the par'- ticular organizations shown and described, but that many-modifications may be madel without departing from the scope of my invention. What I-claimis:

1'. In combination with a pair of electronsec tions'having acommon unbypassed cathode im?- pedance, eachelectron section being provided with respective input andoutput electrodes, means for `rapplying frequency-variable signal energy to the input electrodes of the rst section, means responsive to signal voltage variations' developed across the cathode impedance for varying the space current flow in the second electron. section, an impedance network in circuit with the output electrode of the first electron section, said impedance'network consisting of a resonant system having an amplitude-frequency characteristic provided with maximum andminimumfrequency peaks respectively located on opposite-sides of' the mean frequency of said signalenergy, a separate Vpurely resistive impedancevin circuit with the output electrode of the second'electron section, and means for separately utilizingfpvoltage developed across each ofY said impedance elements. .2.,,In'combination with a pair of electron sec# tions, havingsa commonunbypassed cathode impedance, -each electron section being provided with;V respective input and output electrodes, means for applying frequency-variable signal energy .Ito-the input electrodesof the first section, rneansresponsive to signal voltage variations developed@ r across the cathode l impedance i -for varying the space current flow in the second electron section; an impedance network in circuit with the output electrode of the first electron section, a separate impedance in circuit with the output electrode of the second electron section, means for separately utilizing voltage developed across each of said impedance elements, said first impedance element comprising a resonant network which has a maximum impedance at a frequency located on one side of the mean frequency of applied signal energy, and a minimum impedance at a frequency located on the opposite side of the mean frequency.

' 3. In combination with a pair of electron sections having a common unbypassed cathode impedance, each electron section being provided with respective input arid output electrodes, means for applying frequency-variable signal energy to the input electrodes ofthe first section, means responsive tosignal voltage variations developed across the cathode impedance forvarying the space current flow in the second electron section, an impedance network in circuit with the output electrode of the rst electron section, a separate impedance in circuit with the output electrode of the second electron section, means for separately utilizing voltage developed across each of said impedance elements, both' of said impedance elements being resonant circuits, and one of said resonant circuits being tuned to the mean frequency of applied signal energy.

4. A demodulator network for frequency modulated carrier Wave energy comprising a pair of parallel electronic devices each having respective input and output electrodes, said deviceshaving a common cathode load, means impressing frequency modulated carrier Wave energy upon the input electrodes of one of the devices, said devices being provided with output electrodes each having a respective output impedance, at least one of the routput impedances being a resonant circuit, and the amplitude-frequency characteristics 'of the output impedances being complementary and having a cross-over frequency at the mean frequency of applied frequency modulated energy.

5. In a frequency discriminator networkhaving a pair of input terminals to which are applied frequency-variable waves, an electron discharge system provided with input electrodes and two pair of output terminals, said input terminals being connected to said input electrodes, said network having a pair of independent output impedances of respectively complementary amplitude-frequency characteristics, means connecting each of said impedances in circuit'with a respective pair of said output terminals so as to be traversed by oppositely phased space currents of the system, a separate rectifier connected to each of said output impedances, and means for connecting said rectifiers in polarity opposition to provide a differential rectified voltage output.

6. In combination with a pair of electron sections havingv a common unbypasscd cathode resistor, said electron sections each being provided with respective input and output electrodes, means for applying frequency-variable signal energy to the input electrodes of the first section, means responsive to signal voltage variations developed across the cathode resistor for varying the space current iow in the second electron section, a tuned impedance network in circuit with the output electrode of the first electron section, a separate resistive impedance in circuit with the output electrode of the second electron section, and means for separately utilizing signal voltage developed across each of said impedance elements.

7; Incombination with" a pair of electron sections'having a common unbypassed cathode impedance, said electron sections each being provided with respective input and output electrodes, means for applying frequency-variable signal energy tothe input electrodes of the first section, means responsive to signal voltage variations developed across the cathode impedance for varying the space current ow in the second electron section, an impedance network in circuit with the output electrode of the rst electron section, a separate impedance which is essentially resistive in circuit with the output electrode ofthe second electron section, and means for separately utilizing voltage developed across each of said impedance elements, said first impedance element comprising a resonant network which has a maximum impedance ata frequency located on one side of the mean frequency of applied signal energy, and a minimum impedance at a frequency located on the opposite side -of the mean frequency.

8.V In combination with a pair of electron sections having va common u'ribypassed cathode impedance, said electron sections each being provided with respective input andcutput electrodes, means `for applying frequency-variable signal energyto the input electrodes of the first section, means `responsive to Signal voltage variations developed across the cathode impedance for varying thev space current flow in the second electron section, 'a resonant impedance network in circuit with the output electrode of the rst electron section, a resistor in circuit'with the output electrode ofthe second electron sectiomand means forseparately utilizing signal voltage developed across each of said impedance network and said resistor.

9. In combination with a, pair of electron sections having a common unbypassed cathode resistor, said Velectron sections eachbeing provided with respective input and output electrodes, means 'for applying frequency-variable signal energy to the inputelectrodes of the first section, means responsive to signal voltage variations developed across the cathode resistor for varying the space current now in the second electron section, a tuned impedance network in circuit with the output electrode of the first electron section, a resistor in circuit with the output electrode of the second electron section, means for separately utilizing signal voltage developed across each of said impedance and resistor, said tuned impedance comprising a resonant network which has a maximum impedance at a frequency located on one sideof the mean frequency of applied signal energy and a minimum impedance at a frequency located on theopposite side of the mean frequency.

` `MURRAY G. CROSBY.

' REFERENCES CITED The following references are of record in the ile of this patent:

UNITED STATES PATENTS Number I 

